The duty cycle of a device is the duration the device is active in some period of time. Determining the duty cycle is important in many systems.
In but one example, the applicant's successful LoJack® system includes a vehicle locating unit (VLU) hidden in a vehicle. See U.S. Pat. Nos. 4,177,466 and 4,818,998 incorporated herein by this reference. The VLU includes a receiver and a transmitter (e.g., a transponder). When the vehicle is reported stolen, an RF signal to that effect is transmitted from a base station via one or more relay towers to the VLU in the stolen vehicle to activate the VLU transmitter. Police cars equipped with a vehicle tracking unit (VTU) can detect and track the signal emitted by the VLU in order to retrieve the stolen vehicle.
The battery providing power to the VLU has a finite capacity and power is consumed, inter alia, every time the VLU receiver is energized to listen for activation or other informational signals (e.g., synchronization signals). To save battery power, the VLU receiver is energized only periodically (e.g., every minute or two) and even then only for a very short time, typically between 15 and 50 milliseconds. If no activation or informational signal is received, no action is taken and the VLU receiver is deenergized to save battery power. See, for example, U.S. Pat. No. 6,229,988, U.S. patent application Ser. Nos. 11/131,847, 12/462/949, and U.S. Pat. No. 6,229,988, all incorporated herein by this reference.